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In this article we will discuss about the characteristics of soil moisture.
Soil moisture tension does not indicate neither the moisture content of soil nor the amount of moisture that can be withdrawn for plant use at any particular tension. Knowledge of the amount of moisture a given soil holds at various tensions is necessary to know the amount of water available to plants, water that can be absorbed by the plant and the amount of water required for irrigation.
Soil moisture characteristic curves (moisture extraction curves), which are plot of moisture content versus moisture tension (Fig 4.32), show the amount of moisture a given soil holds at various tensions.
Soil-water characteristic curve (soil-water retention curve, soil-moisture characteristic curve):
1. Describes relationship between soil-water potential and volumetric water content
2. Can be determined by simultaneous measurement of water content and pressure potential
3. As soil drains, the largest soil pores empty first since the capillary forces are smallest in these pores. As the soil drains further, the maximum diameter of the water-filled pores further decreases, corresponding with pores that have decreasing values for the pressure potential (water is held by larger capillary forces)
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4. Soil-water retention is unique and is a function of pore size distribution
5. At a pressure potential of zero, the soil’s volumetric water content is defined as the saturated water content
6. Maximum pressure potential at which soil begins to de-saturate (starting at saturation) is defined as the air entry value of the soil and is determined by the largest pores in the soil.
Soil moisture characteristic curve is more strongly affected by soil texture. Greater the clay content, greater the water content at any particular suction and more gradual the slope of the curve. In a sandy soil, most of the pores are relatively large and once these pores are emptied at a given suction, only a small amount of water remains (Fig 4.33).
Soil structure also affects the shape of the moisture characteristic curve, particularly at the low suction range. Soil compaction decreases the total porosity, especially deceases the volume of large inter-aggregate pores. Hence, the saturation water content and initial decrease of water content at low suction are reduced.
On the other hand, the volume of intermediate size pores is likely to be relatively greater in a compacted soil, while the interaggregate micropores remain unaffected. Hence, the curves for compacted and uncompacted soil may be nearly identical at high suction range (Fig 4.34). At very high suction range, water is held primarily by adsorption and hence retention is a textural than a structural attribute.
Water content and the potential energy of soil-water are not uniquely related because the amount of water present at a given matric potential is dependent on the pore size distribution and the properties of air-water-solid interfaces.
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A soil-water characteristic relationship may be obtained by:
(1) taking an initially saturated sample and applying suction or pressure to de-saturate it (desorption) or by
(2) gradually wetting an initially dry soil (sorption).
These two pathways produce curves that in most cases are not identical (Fig 4.35). The water content in the “drying” curve is higher for a given matric potential than that in the “wetting” branch. This is called hysteresis, defined as the phenomenon exhibited by a system in which the reaction of the system to changes is dependent upon its past reactions to change.
The hysteresis in SWC can be related to several phenomena, including:
1. The ink bottle effect resulting from non-uniformity in shape and sizes of interconnected pores. Drainage is governed by the smaller pore radius r, whereas wetting is dependent on the large radius R (Fig 4.35)
2. Different liquid-solid contact angles for advancing and receding water menisci (Fig 4.35b)
3. Entrapped air in a newly wetted soil
4. Swelling and shrinking of the soil under wetting and drying.
A potentially important aspect of desorption methods under tension is the possibility of liquid displacement (drainage) even in the absence of a continuous gaseous phase due to cavitation initiated by entrapped gas bubbles or the liquid’s own vapor pressure. Surface heterogeneity and impurities in soil and rock water are conducive to lowering the cavitation tension threshold.
Dissolved salts in water increase the force that must be exerted to extract water and thus affect the amount of water available to plants. The increase in tension caused by salts is from osmotic pressure. Plant growth is a function of soil moisture stress, which is the sum of soil moisture tension and osmotic pressure of soil solution. Osmotic pressure developed due to dissolved salts retards uptake of water by plants.
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